The bisulfite reductase (desulfoviridin) from Desulfovibrio vulgaris was purified to homogeneity and a product analysis of its activity was conducted. The substrate, S35-bisulfite was incubated with bisulfite reductase and the products, trithionate, thiosulfate and sulfide were analyzed for the distribution of S35 in these molecules. Trithionate was equally labeled in all three sulfur atoms indicating that they all arose from the substrate. Thiosulfate was labeled in the inner sulfur atom while the outer sulfur atom was unlabeled. The conclusion of these experiments was that the true product of bisulfite reduction was trithionate. Although sulfide contained some radioactivity, the fact that trithionate was formed in quantities 5 to 10 times that of either thiosulfate or sulfide led us to conclude that trithionate was the major enzymatic product.